The invention relates to a planar detector of a device for detecting corpuscular or electromagnetic radiation, utilizing the magnetic superheating properties of small superconducting particles in a magnetic field, with a device for optical registration of the irradiated particles. The invention further relates to a method for manufacturing the planar detector. A similar planar detector is known, for instance, from U.S. Pat. No. 4,149,075.
In medical and physical applications, viewing screens of luminescent crystal layers or image converters with photo cathodes are presently in use for viewing X-ray, diffraction or structure pictures. For the objective reproduction of images which are obtained by central projection by an X-ray tube or as a diffraction image with monochromatic electron radiation, photographic film still plays a dominant role. The properties of these detection means can be subdivided as follows: (a) sensitivity, i.e., the areal intensity just detectable and visible, referred to the incident primary energy; (b) the time resolution of the detection method, which is of importance for following up dynamic processes. For some applications, it is also important that the detector is able to discriminate or sum up the incident radiation as to its energy.
Recently, special properties of superconducting spherical granulates have been used to detect corpuscular or electromagnetic radiation. For this purpose, what is known as the magnetic superheating effect has been utilized. This effect is based on the fact that a smooth superconducting body retains its superconducting properties even if an external magnetic field which is stronger than the equilibrium field acts on it.
This superheating effect will be explained in the following, referring to FIG. 1 of the drawing. In the diagram of the figure, magnetic induction B.sub.c is plotted in arbitrary units for a superconductive material versus the temperature T, above which superconductivity is no longer possible. The phase boundary between superconductivity and normal conduction is shown by a solid line. In the vicinity of the transition point T.sub.c, the magnetic induction is relatively small; the maximum value B.sub.c (0), reached at T=0, is between 15 and 60 mT for known superconductive materials such as tin, lead or mercury.
The effect used in superconducting bolometers on the basis of this relationship for detecting very low radiation intensities is based on a constant temperature T.sub.1 and a magnetic induction B.sub.c1 just below the phase boundary between superconductivity and normal conduction. Due to the absorption of corpuscular or electromagnetic radiation within a given time interval .DELTA.t, the superconducting detector element, for instance, a small sphere of volume V receives the energy .DELTA.E, whereby its temperature is increased by the amount .DELTA.T=.DELTA.E/V.multidot..rho..multidot.c, where .rho. and c are the density and the specific heat of the material, respectively. Since in the temperature range of interest, 2 to 3.5 K., the specific heat c is very small because of Debye's T.sup.3 law and also the volume V can be chosen extremely small, a temperature increase of .DELTA.T=0.2 K. can be produced even with amounts of absorbed energy on the order of .DELTA.E=10.sup.-10 J; these temperature increases are fully sufficient for a transition to the normal conducting phase.
Very small superconducting volumes with a smooth surface, which are a prerequisite for high geometric resolution in the subject of the present invention, have the property of changing into the normal conducting state, with the temperature T.sub.1 held constant, not at the induction B.sub.c1 but not before a value B.sub.sh1, as can be seen from FIG. 1, since the relatively large surface energy, referred to the very small volume, for the present prevents the occurrence of normal conduction. This magnetic induction depending on the type of the superconductor and its geometric dimensions is designated as the superheating induction B.sub.sh, which can be 20 to 100% higher than the value of B.sub.c. While the phase transition from a point 1 to a point 2 shown in FIG. 1 is in principle reversible and both states can be obtained by positive or negative temperature changes .DELTA.T, a state designated with 3 is metastable; i.e., once a normal conducting state 4 situated beyond a phase boundary illustrated in FIG. 1 by a dashed line is reached by a small temperature increase, the magnetic induction must be lowered far below B.sub.c1 in order for the detector element to recover the superconducting state.
The degree of superheating is designated as p=(B.sub.a -B.sub.c)/B.sub.c, where B.sub.a means the applied field and B.sub.c, the thermodynamic field. The temperature increase to be applied for transition to normal conduction can be given theoretically as ##EQU1## where the denominator gives the slope of the B.sub.sh /T curve. By suitable choice of the applied field B.sub.a, the temperature increase .DELTA.T required for the phase transition can be predetermined, which makes discrimination as to the energy possible because .DELTA.T.about..DELTA.E.
These physical facts, known per se, permit the design of a sensitive planar detector if it is possible to indicate for each small superconducting element whether it is in the superconducting or normal conducting state.
Matrix-like detection arrangements are known, in which the superconducting state of a discrete element is ascertained by its diamagnetic behavior, i.e., a magnetic field produced above the detector element by a thin, current carrying wire is displaced if it is superconducting, and delivers a corresponding signal in what is referred to as a read line.
In another method, the planar detector is scanned after the exposure phase by a thin electron beam in raster fashion, whereby each unexposed element is transferred to the normal conducting state. The flipping process is detected by pickup coils comprising one or more smaller sub areas by coordinating and recording each subsignal synchronously with the scanning, i.e., erasing, beam in video equipment, and producing a negative image in this manner. Both recording methods are rather complicated and require extensive shielding measures.
A device known from U.S. Pat. No. 4,149,075 for determining hard radiation, the energy of which is larger than 5 keV, consists of a probe element with individual detector areas which contain free grains of a superconductive material of the first kind and are in a metastable superconducting state in an external magnetic field. As soon as radiation impinges on these grains, they change into the normal conducting state. In this device, the effect of external forces attacking at the grains the transition from the superconducting to the normal conducting state is utilized. Because of this transition, the grains change their position within the specimen upon such a transition under the action, for instance, of the Earth's gravity. In the known device, this geometric change can be determined optically. To this end, the device contains a Stanhope magnifier, which terminates the probe element on its side facing away from the incident radiation (see FIG. 10a). The grains changed to the normal conducting state can be observed in this magnifier as shiny sharp dots, while the remaining superconducting grains are immersed in the optical background.
However, the construction of such a probe element is relatively expensive, as they must comprise a multiplicity of very fine canal-like areas, in each of which one or more superconducting grains are freely arranged. Suitable structures, however, especially with very small cross sections of their canals, can be made only with difficulty. Because of the lower limit of the canal cross sections due to the manufacturing technique, the resolution of the known device for determining hard radiation is accordingly limited. In addition, the distribution only of those elementary rays, the energy of which is relatively large and is above a predetermined threshold, can be determined with the known device. This threshold is furthermore dependent on the size of the individual granules.
It is an object of the present invention to simplify and improve the planar detector known from U.S. Pat. No. 4,149,075 in such a manner that relatively low energy radiation can also be detected with simultaneously relatively high resolution.